1. Field of the Invention
Embodiments of the present invention generally relate to wellbore completion. More particularly, the invention relates to downhole tools. Still more particularly, the invention relates to a downhole vane motor.
2. Description of the Related Art
In a conventional well completion operation, a wellbore is formed by drilling a hole to a predetermined depth to access hydrocarbon-bearing formations. Drilling is accomplished utilizing a drill bit which is mounted on the end of a drill support member, commonly known as a drill string. The drill string is often rotated by a top drive or a rotary table on a surface platform or rig. Alternatively, the drill bit may be rotated by a downhole motor, such as by a positive displacement motor (pdm) or a conventional vane motor.
The conventional vane motor is well known in the art, such as described in U.S. Pat. No. 5,518,379, issued to Harris et al., on May 21, 1996, which is herein incorporated by reference in its entirety. The conventional vane motor and the positive displacement motor are typically powered by a fluid, such as drilling mud, which is pumped through a non-rotating drill string. The conventional vane motor is primarily used in applications involving commingled fluids (nitrogen & drilling mud), high temperature applications, and under balanced drilling applications. Conventional vane motors have an advantage over the positive displacement motor in these instances because they can effectively operate in a corrosive downhole environment. However, these conventional vane type motors have several inherent disadvantages that have limited the use of these tools in the drilling market.
One such disadvantage is that the conventional vane motor has a high output speed. For instance, the conventional vane motor has a rotational speed between 1,500 to 3,000 RPM, as compared to the positive displacement motor which has a rotational speed between 80 to 600 RPM. The high output speed of the conventional vane motor is often times not conducive in removing wellbore material or within a range of speed as dictated by the drill bit designers. The conventional vane motor has a very small displacement volume per revolution resulting in a higher output speed. Therefore, often times, other downhole equipment must be employed, such as a gearbox, to reduce the speed of the conventional vane motor. By employing additional downhole equipment, the overall cost of forming the wellbore is significantly increased.
Another disadvantage is that the conventional vane motor has a low power output. For instance, the conventional vane motor may have a 40% reduction in power as compared to standard pdm of an equivalent size. The conventional vane motor typically includes three required components, a housing, a stator and a rotor. Many times, the size of these components limit the space available for a power fluid chamber, thereby resulting in a small fluid volume chamber. Thus, the low volume characteristics of the conventional vane motor combined with a small surface area per unit pressure results in lower torque output.
Another disadvantage is that the operational life of the conventional vane motor is often times reduced due to the contamination of the internal components by particles circulating through the motor. Additives, such as abrasive particles, are typically added to the drilling mud to maintain the drilling mud properties. These particles must be filtered and prevented from circulating through the conventional vane motor otherwise seals and sealing surfaces will wear at an accelerated rate causing component damage. Typically, additional filter equipment must be installed on the surface along with additional downhole filters to properly filter the drilling fluid; thus, adding to operational costs and introducing additional maintenance and reliability issues.
Another disadvantage is that the conventional vane motor includes many complex parts resulting in a decrease in their reliability and increase in their maintenance costs. For instance, in addition to the housing, the stator, and the rotor as previously discussed, often times the conventional vane motor includes an elaborate shimming arrangement for maintaining the alignment and the tolerances between the components. Furthermore, the time required to service the conventional vane motor is typically 2 to 3 times the standard time that is required to service the pdm motor. This is partly due to the tight tolerances and fine adjustments that make the conventional vane motor impractical to service in a shop environment and in remote locations where tooling and expertise are limited. Drilling operators have dealt with the reliability issues by providing the customer with redundant vane motors. In the event that a vane motor fails, several backup vane motors are made available on location.
Another disadvantage is that the conventional vane motor does not tolerate misalignment due to bending or side load conditions. A large portion of the current drilling market cannot be penetrated with the vane motor technology because the risk factors are high for component failure in a side load condition. For instance, casing exits, side tracks, and special applications must utilize pdm technology to complete jobs. Often times, the pdm is not suited for the application due to high temperature, pressure, or nitrogen requirement.
Various designs have been developed to improve the conventional vane motor. For instance, one design uses rolling elements as sealing members as described in U.S. Pat. No. 6,302,666, issued to Gupping et al., on Oct. 16, 2001, which is herein incorporated by reference in its entirety. In another design, a motor having a stator with a rod recess formed therein is used in conjunction with a rod to act as a valve for opening and closing an inlet/exhaust port, as described in U.S. Pat. No. 5,833,444, issued to Harris et al., on Nov. 10, 1998, which is herein incorporated by reference in its entirety. However, these designs do not address the reliability and performance issues of the conventional vane motor.
A need therefore exists for a vane motor having a lower output speed. There is a further need for a vane motor with an increased power output. There is yet a further need for a simple vane motor that is reliable. Further, there is a need for a vane motor that includes a self cleaning means, thereby minimizing component damage. Furthermore, there is a need for an improved vane motor.